This project deals with molecular mechanisms underlying Avian Leukosis Virus (ALV) induced neoplasms including bursal lymphomas in chickens. The integrated exogenous provirus of ALV and adjacent host sequences in clonal neoplasms will be cloned in prokaryotic vectors using recombinant DNA technology and detailed studies of the structure and function of these sequences will be carried out. Evidence of recombination with host cell sequences will be sought and integration site of ALV provirus in different normal and neoplastic tissues will be compared. The sequences in cellular DNA preparation from ALV induced neoplasms which transform NIH/3T3 mouse cells in transfection experiments will be characterized with respect to known putative transforming genes in chicken cells and an attempt made to isolate these sequences by molecular cloning techniques. The genetic elements of ALV responsible for its oncongenic properties will be identified by biochemical substitution of allelic segments of a non-oncogenic endogenous avian retrovirus.